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Chapter 6 Sedimentary Rocks. What is a sedimentary rock?. Sedimentary rocks are products of mechanical and chemical weathering. Generally formed by the deposition, burial , compaction, and cementation of sediments. They account for about 5% (by volume) of Earth’s outer 10 miles - PowerPoint PPT Presentation
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Chapter 6 Sedimentary Rocks
What is a sedimentary rock?
Sedimentary rocks are products of mechanical and chemical weathering.
Generally formed by the deposition, burial , compaction, and cementation of sediments.
They account for about 5% (by volume) of Earth’s outer 10 miles
Contain evidence of past environments• Provide information about sediment transport• Often contain fossils
What is a sedimentary rock?
Sedimentary rocks are important for economic considerations because they may contain
• Coal• Petroleum and natural gas• Sources of iron, aluminum, and manganese
Turning sediment into rock
Many changes occur to sediment after it is deposited
Diagenesis = chemical, physical, and biological changes that take place after sediments are deposited
• Occurs within the upper few kilometers of Earth’s crust
Turning sediment into rock
Diagenesis• Includes
• Recrystallization – development of more stable minerals from less stable ones
• Lithification – sediments are transformed into solid rock by
• Compaction and cementation• Natural cements include calcite, silica, and
iron oxide
Types of sedimentary rocks
Sediment originates from mechanical and/or chemical weathering
Rock types are based on the source of the material
• Detrital (Clastic) rocks – transported sediment as solid particles, made of broken pieces)
• Chemical (& biochemical) rocks – sediment that was once in solution and precipitates.
Types of Sedimentary Rocks: Clastic (also called Detrial)—made
of broken pieces of other rocks
Chemical—minerals dissolved in lakes, seas, or underground water
Types of Sedimentary Rocks:
Mineral crystals are made as the shallow water that has flooded the bottom of Death Valley evaporates. Click on image for full size (66K JPG)
Courtesy of Martin Miller, University of Oregon
Biochemical (Organic)—remains of plants and animals are deposited in thick layers
Examples Fossil rich limestone is made from the
shells of ocean animals; used to make chalk
Types of Sedimentary Rocks:
Detrital sedimentary rocks
The chief constituents of detrital rocks include
• Clay minerals• Quartz• Feldspars• Micas
Particle size is used to distinguish among the various rock types
Detrital sedimentary rocks
Common detrital sedimentary rocks• Shale
• Mud-sized particles in thin layers that are commonly referred to as laminea
• Most common sedimentary rock
• Sandstone• Composed of sand-sized particles( well sorted to
poorly sorted)• Forms in a variety of environments• Quartz is the predominant mineral
Shale with plant remains
Figure 7.2
Detrital sedimentary rocks
Alternating sequences of shale and sandstone exposed in the Grand Canyon. Shale cannot support steep cliffs or form erosion
Quartz sandstone
Figure 7.4
sandstone
Arkose (with feldspar) Quartz Sandstone
Detrital sedimentary rocks
• Conglomerate and breccia• Both are composed of particles greater than
2mm in diameter
• Conglomerate consists largely of rounded gravels
• Breccia is composed mainly of large angular particles
Conglomerate
Figure 7.6
Breccia
Chemical sedimentary rocks
Consist of precipitated material that was once in solution
Precipitation of material occurs by• Inorganic processes• Organic processes (biochemical origin)
Chemical sedimentary rocks
Common chemical sedimentary rocks• Limestone
• Most abundant chemical rock
• Composed chiefly of the mineral calcite
• Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms)
• Inorganic limestones include travertine and oolitic limestone
Coquina
Figure 7.9
Fossiliferous limestone
Limestones (organic) ChalkCoquina
Fossiliferous
Stalactites hanging (tight) to the ceiling of a cave.
Limestones (inorganic) Travertine
LimestonesOolitic (inorganic)
Chemical sedimentary rocks
Common chemical sedimentary rocks• Dolostone
• Typically formed secondarily from limestone
• Chert• Composed of microcrystalline quartz
• Varieties include flint and jasper (banded form is called agate)
Agate
Figure 7.12 A
Sedimentary Rocks
A. Quartz (SiO2)“flint”, “chert”
Chemical sedimentary rocks
Common chemical sedimentary rocks• Evaporites
• Evaporation triggers deposition of chemical precipitates
• Examples include rock salt and rock gypsum
Chemical sedimentary rocks
Common chemical sedimentary rocks• Coal
• Different from other rocks because it is composed of organic material
• Stages in coal formation (in order)• 1. Plant material• 2. Peat• 3. Lignite• 4. Bituminous
Stagesof
coal formation
Figure 7.15
Stagesof
coal formation
Figure 7.15
Classification of sedimentary rocks
Sedimentary rocks are classified according to the type of material
Two major groups• Detrital• Chemical
Classification ofsedimentary rocks
Two major textures are used in the classification of sedimentary rocks
• Clastic• Discrete fragments and particles
• All detrital rocks have a clastic texture
• Nonclastic• Pattern of interlocking crystals
• May resemble an igneous rock
Identification of sedimentary rocks
Figure 7.17
3 Types of Sedimentary Rock1. Clastic
Formed from the deposition, burial, compaction, and cementation of sediments (fragments of other rock)
Examples
Grain
Size
Composition
3 Types of Sedimentary Rock
2. Chemical (crystalline) Precipitates – minerals fall out of solution
when the water chemistry changes Evaporates – minerals left behind when
water evaporates
Sedimentary environments
A geographic setting where sediment is accumulating
Determines the nature of the sediments that accumulate (grain size, grain shape, etc.)
Sedimentary environments
Types of sedimentary environments• Continental
• Dominated by stream erosion and deposition
• Glacial
• Wind (eolian)
• Marine• Shallow (to about 200 meters)
• Deep (seaward of continental shelves)
Sedimentary environments
• Transitional (shoreline)• Tidal flats
• Lagoons
• Deltas
Sedimentaryenvironments
Figure 7.19 (left)
Sedimentaryenvironments
Figure 7.19 (right)
Sedimentary environments
Sedimentary facies• Different sediments often accumulate
adjacent to one another at the same time• Each unit (called a facies) possesses a
distinctive set of characteristics reflecting the conditions of a particular environment
• The merging of adjacent facies is a gradual transition
Sedimentary facies
Figure 7.20
Sedimentary structures
Provide information useful in the interpretation of Earth history
Types of sedimentary structures• Strata, or beds (most characteristic of
sedimentary rocks)• Bedding planes that separate strata• Cross-bedding
Sedimentary structures
Types of sedimentary structures• Graded beds• Ripple marks• Mud cracks• Fossils
Characteristics of Sedimentary Rocks
May stratified because the sediments are laid down in horizontal layers called strata. ( one layer is called stratum)
May also contain fossil i.e: remains, prints or other indications of plants & animals found buried in rocks
Dipping sedimentary layers of rock, Rocky Mountains, Canada.
http://www.geog.ouc.bc.ca/physgeog/contents/10f.html
Fossils: Evidence of past life
By definition, fossils are the traces or remains of prehistoric life now preserved in rock
Fossils are generally found in sediment or sedimentary rock (rarely in metamorphic and never in igneous rock)
Fossils: Evidence of past life
Geologically fossils are important for several reasons
• Aid in interpretation of the geologic past• Serve as important time indicators• Allow for correlation of rocks from different
places
Features of Sedimentary Rocks
Stratification (bedding) is when layers of sedimentary rocks form stacked on top of each other
Features of Sedimentary Rocks
Ripple Marks are sand patterns formed by the action of winds, streams, waves, or currents
Features of Sedimentary Rocks
Mud Cracks develop in clay when it dries out and hardens into rock.
End of Chapter 6